Process of manufacturing metallic nano-scale powders

ABSTRACT

A process for synthesizing metal submicron and nano-scale powders for use in articles of manufacture. In a suitable reactor, single metal or multiple metal complexes are heated to a temperature whereby, upon contact with hydrogen gas, an exothermic reaction begins. The further temperature rise in response to the exothermic reaction is minimized by reducing the external heat input, thereby minimizing the agglomeration or sintering of the metal nano-scale particles resulting from the process. Preferably, after drawing a vacuum on the metal complexes in the reactor, the hydrogen is introduced at above, equal to or below ambient pressure and the reaction is purposely made slow to prevent agglomeration or sintering.

This application claims the benefit of provisional patent applicationNo. 60/792,855, filed Apr. 18, 2006.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods of producing sub-micronpowders, nanoparticles and articles made therefrom.

BACKGROUND OF THE INVENTION

Powders are used in numerous applications. Powders are the buildingblocks of catalytic, electronic, telecommunication, electrical,magnetic, structural, optical, biomedical, chemical, thermal, andconsumer goods. On-going market demands for more efficient, reliable,smaller, faster, superior, and more portable products have demandedminiaturization of numerous products. This, in turn, has demandedminiaturization of the building blocks, i.e. the powders. Nano-scale (ornanosize, ultra-fine) powders, with a size of 10 to 100 times smallerthan conventional micron size powders, enable quality improvement anddifferentiation of product characteristics at scales currentlyunachievable by commercially available micron-sized powders.

Nano-scale powders, in particular, are a novel family of materials whosedistinguishing features include a domain size so small that sizeconfinement effects become a significant determinant of the materials'performance. Such confinement effects can, therefore, lead to a widerange of commercially important properties. Thus, nano-scale powdersoffer an extraordinary opportunity for design, development, andcommercialization of a wide range of devices and products for variousapplications. Furthermore, since they represent a whole new family ofmaterial precursors where conventional coarse-grain physiochemicalmechanisms are not applicable, these materials offer unique combinationof properties that can enable novel and multifunctional components ofunmatched performance. Other examples of sub-micron and nano-scalepowder applications are described in U.S. Pat. No. 5,984,997, which ishereby incorporated by reference along with the references containedtherein.

Traditional methods of producing fine metal powders chiefly involveplasma reactions, such as the process described in European PatentApplication Publication EP1619000169, which is hereby incorporated byreference, or the condensation from gas and liquid phase described inU.S. Patent Application Publication No. 20050277297, which is herebyincorporated by reference. These known methods require relatively hightemperatures exceeding several hundred degrees Celsius so that metalgrain sizes can rapidly grow during sintering. Moreover, these methodsare inefficient and do not typically produce nano-scale powders. Thesemethods also consume a large amount of energy, and, therefore areexpensive. Likewise, the production of metal powders from hydrogenreduced oxides is a well known technique; however, this process suffersfrom similar drawbacks. The great expense associated with traditionaltechniques of producing fine metal particles limits the applications inwhich the metal particles can be used.

U.S. Pat. No. 3,955,961 discloses processes for reducing certain metalcarboxylates with hydrogen or carbon monoxide under low moistureconditions, relatively low temperatures and preferably high pressures.The patent teaches starting with relatively large (2.5 cm) pellets,increasing the hydrogen pressure to speed up the reaction rate andallowing the exothermic reaction to raise the temperature well above thebeginning temperatures to thereby increase the production rate anddecrease cost. The patent issued at a time (1976) long before thepossible manufacture of submission or nano-particles was seriouslycontemplated.

SUMMARY OF THE INVENTION

In general, the present invention provides a process for synthesizingmetal powders comprising the step of heating a solid metal complex to alow temperature above ambient, the step of chemically reacting the metalcomplex with hydrogen in a low temperature environment, and the step ofcarefully limiting the temperature rise caused by the exothermicreaction ensuing to thereby minimize the agglomeration or sintering ofthe metal particles resulting from the process.

Another aspect of the present invention provides particles produced byprocessing for at least one substantially pure metal submicron ornano-scale powder for inclusion in subsequent products.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (1 μm reference).

FIG. 2 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (300 nmreference).

FIG. 3 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (3 μm reference).

FIG. 4 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (300 nmreference).

FIG. 5 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (1 μm reference).

FIG. 6 is a scanning electron microscope picture of exemplary particlessynthesized using the process of the present invention (1 μm reference).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Definitions

As used herein, “nano-sized”, “nano-scale”, or “nanoparticle” refers toa particle with an average particle size of about 100 nm or less (e.g.,less than about 95 nm, less than about 80 nm, less than about 70 nm, orless than about 65 nm). The average particle size for nanoparticlesrefers to at least one dimension of the resulting metallic particles,which may be of any shape, e.g., spherical, eliptical, rectangular orirregular, having an average value of about 100 nm or less. Somenanoparticles may have more than one dimension which, on average, has avalue of about 100 nm or less. The average dimension can be referred toas a D₅₀ value.

As used herein, “sub-micron” refers to a particle with an averageparticle size of more than 100 nm to about 1 μm (e.g., more than 200 nmto about 900 nm or more than 300 nm to about 700 nm). The averageparticle size refers to the dimensions of the resulting metallicparticles, which may be of any shape, e.g., spherical, eliptical,rectangular or irregular. The average dimensions of sub-micron metallicparticles are between about 100 nm to about 1 μm. The average dimensioncan be referred to as a D₅₀ value.

As used herein, “micro” refers to a particle with an average particlesize of more than 1 μm to about 900 μm.

As used herein, “low temperature” refers to temperatures from about 20°C. to about 700° C.

As used herein, a “metal complex” refers to a compound having at leastone metal atom wherein the metal atom is bonded to one or more ligands.Metal complexes comprising more than one metal atom may comprise two ormore metal atoms of the same element or they may comprise two or moremetal atoms of the different elements. Metal complexes include metalsalts and metal chelates. Examples of metal complexes include metalcarbonates, metal citrates, metal oxalates, metal carbazides, metalglycines, metal hydroxides, or the like.

As used herein, a “metal atom” is an electropositive atom having anatomic number of 3 to 94. Examples of metal atoms include, withoutlimitation Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Mo, Ru, Rh, Pd, Ag, Cd,In, Sn, W, Re, Os, Ir, Pt, Au, Hg, Ti, Pb, Bi, or the like.

As used herein, “metal powder” refers to powders comprising a pureelemental metal (e.g., pure Au, Ag, Pt, Fe, Ni, or the like) and metalalloys, wherein a metal alloy is a combination, either in solution orcompound, of two or more elemental metals, and where the resultantmaterial has metallic properties. Examples of metal alloys includes,without limitation, bronze, brass, aluminum alloys, nickel alloys,titanium alloys, iron alloys (e.g., stainless steel), magnesium alloys,or the like.

As used herein, a “reducing agent”, a “reductant”, or a “reducer” is asubstance that chemically reduces other substances by donating anelectron or electrons, and reduction describes the gain of one or moreelectrons by a molecule, atom or ion.

As used herein, an “oxidizing agent”, an oxidizer, or an “oxidant”refers to substances that have the ability to oxidize other substancesby removing an electron or electrons, and oxidation describes the lossof one or more electrons by a molecule, atom or ion.

As used herein, “average particle size” refers to the length of at leastone dimension of at least one side of a particle as determined usingScanning Electron Microscopy, Transmission Electron Microscopy, and/orLight Scattering techniques known to those skilled in the art.

As used herein, “surface area” refers to the summation of the areas ofthe exposed sides of an object. The surface area of fine powders andnano-scale powders of the present invention was determined using N₂ gasadsorption to calculate the BET surface area. The BET equation forcalculating surface area is expressed by (1):

$\begin{matrix}{\frac{1}{\upsilon\left\lbrack {\left( {P_{0}/P} \right) - 1} \right\rbrack} = {{\frac{1}{\upsilon_{m}c}\left( \frac{P}{P_{0}} \right)} + \frac{1}{\upsilon_{m}}}} & (1)\end{matrix}$wherein P and P₀ are the equilibrium and the saturation pressure ofadsorbates at the temperature of adsorption, ν is the adsorbed gasquantity (for example, in volume units), and ν_(m), is the monolayeradsorbed gas quantity. c is the BET constant, which is expressed by (2):

$\begin{matrix}{c = {\exp\left( \frac{E_{1} - E_{L}}{RT} \right)}} & (2)\end{matrix}$wherein E₁ is the heat of adsorption for the first layer, and E_(L) isthat for the second and higher layers and is equal to the heat ofliquefaction.

Equation (1) is an adsorption isotherm and can be plotted as a straightline with 1/ν[(P_(o)/P)−1] on the y-axis and P/P₀ on the x-axisaccording to experimental results. This plot is called a BET plot. Thelinear relationship of this equation is maintained only in the range of0.05<P/P₀<0.35. The value of the slope and the y-intercept of the lineare used to calculate the monolayer adsorbed gas quantity ν_(m) and theBET constant c.

The BET method is widely used in surface science for the calculation ofsurface areas of solids by physical adsorption of gas molecules. A totalsurface area S_(total) and a specific surface area S are evaluated bythe following equations:

$\quad\begin{matrix}{S_{total} = \frac{\left( {\upsilon_{m}N_{s}} \right)}{M}} \\{S = \frac{S_{total}}{a}}\end{matrix}$wherein N is Avogadro's number, s is adsorption cross section, M ismolecular weight of adsorbate, and a is the weight of the sample solid.

II. Synthesis of the Submicron and Nano-Scale Powders

The present invention provides a process for synthesizing metal powdersincluding the step of heating one or more metal complexes to asufficient but low temperature above ambient, the step of contacting themetal complex with hydrogen gas (H₂) at the low temperature and uponexothermic reaction the step of carefully limiting the temperature risecaused by the exothermic reaction to thereby minimize the agglomerationor sintering of metallic particles produced by the reaction. Thereaction is continued for a sufficient period of time with the hydrogengas at a suitable pressure, whereby the hydrogen gas contacts the metalcomplex at the controlled low temperature for a sufficient period oftime to form metal nanoparticles and/or sub-micron particles. The objectof the process is to produce substantially pure metal nanoparticlesand/or sub-micron particles. Temperatures that are suitable for thepresent invention are sufficiently controlled so that when the metalcomplex contacts the hydrogen gas for an adequate period of time, metalnanoparticles and/or sub-micron particles are formed but largerparticles are minimized. Periods of time suitable for the hydrogen tocontact the metal complex have a duration, at the expiration of which,the metal complex and the hydrogen gas have been in contact to formsubstantially pure metal nanoparticles and/or sub-micron particles fromsubstantially all of the metal complex present. Prior to contacting withhydrogen gas preferably, a vacuum is drawn on the metal complexes in thereactor. Suitable pressures for hydrogen gas include pressures that areabove atmospheric pressure, substantially equal to atmospheric pressure,or below atmospheric pressure.

Substantially pure metal powders synthesized using the process of thepresent invention include pure elemental metal powders (e.g., goldpowder, nickel powder, copper powder, iron powder, or the like, eachhaving a purity of greater than about 80%, greater than about 90%,greater than about 95%, or greater than about 99%) and powders of metalalloys (e.g., brass powder, bronze powder, or titanium alloy powder,each having a purity of greater than about 80%, greater than about 90%,greater than about 95%, or greater than about 99%). In fact, the metalpowders produced using the process of the present invention do not havesubstantially passivated surfaces if the process is allowed to continueto substantial completion.

Without intending to be limited by theory, it is theorized that when themetal complex reacts with hydrogen gas (H₂) at a low controlledtemperature, each metal cation is reduced by the hydrogen gas to formthe neutrally charged metal while the metal complex anion and/thehydrogen gas are oxidized and expelled from the reactor.

In examples of the embodiment, the present invention provides a processfor synthesizing substantially pure elemental metal nanoparticlescomprising the step of heating a metal complex in a reactor to atemperature of 700° C. or less and drawing a vacuum thereon (e.g., fromabout 80° C. to about 300° C., from about 150° C. to about 300° C., orfrom about 240° C. to about 260° C.; or about 300° C. or less, 290° C.or less, 280° C. or less, or about 275° C. or less); and the step ofcontacting the metal complex with hydrogen gas having a suitablepressure in the reactor at a controlled reaction temperature of lessthan 700° C. for a period of about 20 minutes to about 10 hours (e.g.,about 1 hours to about 5 hours, about 2 hours to about 4.5 hours; or fora period of about 1 hour or more, 2 hours or more, 3 hours or more, 4hours or more, or the like).

In another embodiment, the present invention provides a process forsynthesizing substantially pure elemental metal submicron ornanoparticles comprising the step of heating a mixture of two or moremetal complexes, the step of contacting the mixture with hydrogen,wherein the metal complexes each have a metal atom of the same element(e.g., Ni, Cu, Co, Fe, or the like), but the metal complexes havediffering ligands, i.e., at least one ligand attached to one metal atomof one metal complex being different from ligands on the metal atoms ofother metal complexes. For example, in one specific embodiment, aprocess for synthesizing substantially pure nickel (Ni) nanoparticlesand/or sub-micron particles comprises the step of heating a mixture oftwo metal complexes to a temperature of about 150° C. to about 300° C.,wherein the mixture comprises a first metal complex of nickel hydroxideand a second metal complex of nickel carbonate hydrate; the step ofcontacting the mixture with hydrogen gas having a suitable pressure inan environment having a controlled temperature of less than 300° C. andthe step of maintaining the controlled temperature for a period of about3 hours to about 4 hours.

In another embodiment, the present invention provides a process forsynthesizing a powder having a mixture of substantially pure elementalmetal submicron or nanoparticles comprising the step of heating amixture of two or more metal complexes to a sufficient temperature forreaction with hydrogen gas, the step of contacting the mixture withhydrogen gas and the step of limiting the temperature of the resultingexothermic reaction to minimize agglomeration or sintering of the metalparticles formed from the reaction, wherein each of the metal complexeshas at least one unique metal atom, i.e., at least one metal atom of anelement that differs from at least some of the other metals in the metalcomplexes, for a sufficient period of time to substantially complete thereaction of hydrogen gas with the metal complexes.

In several embodiments, the present invention provides a method forsynthesizing metal alloy nanoparticles comprising the step of heating amixture of two or more metal complexes to a sufficient temperature, andthe step of contacting the mixture with hydrogen gas, followed bycontrolling the reaction temperature wherein each of the metal complexeshas at least one unique metal atom, i.e., at least one metal atom of anelement that is not present in the other metal complex, for a sufficientperiod of time.

In the alternative embodiments, the metal powders are synthesized byheating one or more metal complexes, such as metal carbonates, metalcitrates, metal oxalates, metal carbazides, metal glycines, metalhydroxides, or the like, wherein the complexes include one or moremetals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Mo, Ru, Rh, Pd, Ag,Cd, In, Sn, W, Re, Os, Ir, Pt, Au, Hg, Ti, Pb, Bi or the like to atemperature of less than 700° C.; and reacting the metal complex withhydrogen at a temperature of less than 700° C. under a carefullycontrolled time/temperature relationship. The resulting metal powdercomprises nanoparticles and/or sub-micron particles. Moreover, theresulting metal powder can also have a surface area of more than 3 m²/g(e.g. more than 5 m²/g, more than 10 m²/g, more than 20 m²/g, more than30 m²/g, more than 50 m²/g, or more than 80 m²/g). Surface area iscritical for use as a catalyst for example.

Nanoparticles synthesized using the process of the present invention canundergo further processing, such as sintering, to form sub-micronparticles or micro particles, if so desired.

Another aspect of the present invention provides a process of directlysynthesizing substantially pure sub-micron metal particles comprisingthe step of heating a metal complex to a sufficiently high temperature(e.g., less than 300° C., less than 350° C., less than 450° C., lessthan 550° C., or less than 750° C.) and the step of contacting the metalcomplex with hydrogen at the sufficiently high temperature for asufficient period of time (e.g., about 1 hour to about 5 hours, about 2hours to about 4.5 hours; or for a period of about 1 hour or more, 2hours or more, 3 hours or more, 4 hours or more, or the like) such thatthe interaction between the hydrogen and the metal complex formssubstantially pure sub-micron metal particles. The higher temperatureprovides for the partial agglomeration or sintering to the larger sizeparticles as desired.

A further aspect of the present invention broadly provides a process ofsynthesizing substantially pure sub-micron elemental metal particlescomprising the step of heating at least one metal complex, and the stepof contacting the metal complex with hydrogen at a sufficiently high butcontrolled temperature for a sufficient period of time such that theinteraction between the hydrogen and the metal complex formssubstantially pure sub-micron elemental metal particles.

Another aspect of the present invention provides a method ofsynthesizing substantially pure sub-micron metal alloy particlescomprising the step of heating a mixture of 2 or more metal complexes,and the step of contacting the mixture with hydrogen at a sufficientlyhigh but controlled temperature for a sufficient period of time suchthat the interaction between the hydrogen and the metal complex formssubstantially pure sub-micron metal alloy particles.

Metal powders synthesized using the process of the present invention(e.g., the production of metal nanoparticles and/or sub-micronparticles) can undergo further processing (such as sintering) to modifythe powder properties, such as increase the particle size, impart amagnetic carrier, or the like. Moreover, powders synthesized using theprocess of the present invention can be molded or cast into forms usingknown manufacturing methods, or the powders can be mixed with polymers(e.g., thermoplastics, thermosets, elastomers, or the like) or othermetals and processed by molding or casting into forms. Powders of thepresent invention can be mixed with adhesives, bonding materials,molding compounds, or fluid carriers such as solvents, paints, surfacetreatments, atmospheric gases, or the like.

III. Other Embodiments

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

1. A process for synthesizing a substantially pure metal powdercomprising the steps of: heating a mixture of two or more metalcomplexes to a low temperature, the two or more metal complexes having ametal atom of the same element, but the two or more metal complexeshaving differing ligands, contacting the mixture with hydrogen gas atthe low temperature causing an exothermic reaction, and controlling thetemperature rise of the exothermic reaction to minimize theagglomeration of the metallic particles produced by the reaction.
 2. Theprocess of claim 1, wherein the two or more metal complexes includenickel hydroxide and a nickel carbonate hydrate.
 3. The process of claim2, wherein the metal complexes are heated to a temperature of less than700° C.
 4. The process of claim 2 wherein the metal powder comprisesnickel submicron particles.
 5. The process of claim 1, wherein the metalpowder comprises nickel nanoparticles.
 6. The process of claim 1,wherein the metal powder has a surface area of 3 m²/g or more.
 7. Theprocess of claim 1, wherein a vacuum is drawn on the metal complexesprior to contact with hydrogen gas.
 8. The process of claim 1, whereinthe metal complexes are selected from the group consisting of a metalcitrate, a metal oxalate, a metal carbazide, a metal glycine, and ametal hydroxide.
 9. The process of claim 8, wherein the metal complexesinclude at least one metal selected from the group consisting of Cr, Mn,Fe, Co, Ni, Cu, Zn, Ga, Ge, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, W, Re, Os,Ir, Pt, Au, Hg, Ti, Pb, and Bi.
 10. The process of claim 1, wherein avacuum is drawn on the metal complex prior to contact with hydrogen gas.11. The process of claim 1, wherein the step of controlling thetemperature rise includes substantially maintaining the low temperature.12. A process for synthesizing a metal powder comprising the steps of:heating a mixture of two or more metal complexes to a temperature below700° C., the two or more metal complexes have a metal atom of the sameelement, but the two or more metal complexes have differing ligands,contacting the metal complexes with hydrogen gas causing an exothermicreaction, and limiting the temperature rise caused by the exothermicreaction to minimize the agglomeration of metallic particles produced bythe reaction.
 13. The process of claim 12, wherein the metal complexesare selected from the group consisting of a metal citrate, a metaloxalate, a metal carbazide, a metal glycine, and a metal hydroxide. 14.The process of claim 13, wherein the metal complexes include at leastone metal selected from the group consisting of Cr, Mn, Fe, Co, Ni, Cu,Zn, Ga, Ge, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, W, Re, Os, Ir, Pt, Au, Hg,Ti, Pb, and Bi.
 15. The process of claim 12, wherein a vacuum is drawnon the metal complex prior to contact with hydrogen gas.
 16. The processof claim 12, wherein the metal powder comprises nanoparticles.
 17. Theprocess of claim 12, wherein the metal powder comprises sub-micronparticles.
 18. The process of claim 12, wherein the metal powder has asurface area of 3 m²/g or more.
 19. The process of claim 12, wherein themetal powder comprises sub-micron particles.
 20. The process of claim12, wherein the step of limiting the temperature rise includessubstantially maintaining the temperature.